Determining the glyphosate tolerance of eastern white cedar: First year post-treatment results

Abstract

Little is known about the silvicultural traits of eastern white cedar (Thuja occidentalis L.), and especially the tolerance of seedlings to herbicides. A study was established to determine the sensitivity of cedar seedlings to timing and concentration of glyphosate applications. Glyphosate was applied to seedlings at three concentrations (1.04, 2.07, and 4.14 acid equivalent (ae) kg ha-1) at three times (July 28, August 10 and August 31) and their survival and growth was compared with that of seedlings in three control treatments (competition free, always competition, and competition free after an August 10 application of 2.07 kg ha-1 glyphosate) for a total of 12 treatments. Damage to foliage increased (r2 = 0.99) and first-year volume growth decreased (r2 = 0.91) with increasing glyphosate concentration. Glyphosate applied at 4.14 kg ha-1 killed 9% of the seedlings; even the lowest concentration of glyphosate (1.04 kg ha-1) reduced first-year volume growth more than continuous herbaceous competition. Application timing did not affect the amount of foliar damage or the growth of seedlings. However, in the winter following the treatments, snow damage was greater in the weed-free control seedlings than in all other treatments.